1. Field of the Invention
The invention relates to gas turbine engine fan and compressor blades and vanes and, more particularly, to vibration dampers for the airfoils of the blades and vanes.
2. Description of Related Art
Modern aircraft gas turbine engine fan and compressor blades having low aspect ratio airfoils are often subject to high stage loading and may experience chordwise vibration at lower frequencies. The problem is exasperated in the case of wide chord and twisted fan blades which tend to be wider at the tip and more narrow at the hub or base of the blase's airfoil. The potential for resonance crossings occurring at high engine speeds and consequently high energy increases and can cause significant high cycle fatigue problems. These can result in liberation of portions of the airfoil.
Stripe mode resonance can present a significant problem in the design and development of gas turbine engine fan and compressor blades. Stripe mode resonance is a plate deformation vibratory mode, a high frequency resonance phenomenon associated with the aerodynamic wakes generated by rows of airfoils upstream of the blade row experiencing the stripe mode excitation. It is very local in both stress and deformation, being located primarily in a radially outward tip portion of the blade which undergoes predominantly chordwise bending. In contrast, low frequency modes of flexure and torsion extend over a large portion of the blade and produce significant stresses in a radially inward portion of the airfoil.
Higher order modes of vibration are more difficult to damp. Upstream airfoils within a multiple stage rotor assembly, for example, can create aerodynamic wakes that cause downstream airfoils to experience higher order modes of vibration. This vibration can result in chordwise bending, particularly, in radially outer portions of the airfoil in a non-symmetrical pattern and is accordingly difficult to predict in terms of magnitude and position.
What is needed, therefore, is an apparatus and/or a method for damping higher order modes of vibration in a blade of a rotor assembly. U.S. Pat. No. 5,498,137 discloses an airfoil with a recess in the form of a pocket formed in a chordwise surface. A damper is received within the pocket between an inner surface of the pocket and a pocket lid attached to the airfoil by conventional attachment apparatus and contoured to match the curvature of the airfoil. Frictional wear is a concern that is overcome by biasing the damper. One problem due to such a design is the stress concentration, often referred to as Kt, around the recess or pocket. The recess or pocket serves as a stress concentrator which reduces the useful life of the blade. Another problem of such designs is fretting which is a distinguished from wear. Fretting due to the damper vibrating against friction producing surfaces causes tiny cracks to form in these surfaces. This in and of itself is not a serious problem but when the cracks begin to propagate, the resultant crack growth can cause failure of a portion of the blade and/or reduce its useful life.
Therefore, it is highly desirable to provide an easily manufactured damping apparatus for the airfoils of such blades which will also overcome wear and fretting problems due to friction which such dampers utilize to damp vibration of blades. It is also highly desirable to provide a damping apparatus which doesn't significantly reduce the life of the airfoil. It is also desirable to manufacture a blade with a recess and a damper received therein which is less prone to failure due to stress concentration around the recess.